The determination of molecular weight and sedimentation coefficient via measurement of sedimentation equilibrium is a classical method of macromolecular chemistry and is well described in many textbooks. Until recently the utilization of this method required the use of large and expensive analytical centrifuges which demanded considerable expertise. Recently the molecular weights of proteins via sedimentation equilibrium have been successfully measured using a relatively inexpensive air-driven centrifuge, namely, the Beckman Airfuge. In order to calculate the molecular weight it is necessary to measure the gradient of solute concentration in the centrifuge tube at sedimentation equilibrium. This was accomplished by fractionating the solution in the tube into layers according to vertical position (typically, 10 layers in a 100 microliters sample), and measuring the average concentration of solute in each layer. This operation is tedious and requires manual dexterity, which may explain why the use of the Airfuge for the determination of molecular weights has not become widespread. Moreover, the resolution of the data obtained via manual fractionation is insufficient for precise determination of sedimentation coefficient. There is a definite need for a system to automatically measure the gradient of solute concentration in the centrifuge tube and to interpret the measured gradient with minimal user manipulation or other intervention, with smaller samples than heretofore required, and using generally available preparative ultracentrifuges, as well as the Beckman Airfuge.
There is also a definite need for a system enabling the concentration of solute to be automatically determined as a function of position in the centrifuge tube, without the need to fractionate the contents of the tube, which would also enable completely automatic determination of molecular weight and/or sedimentation coefficient.
A preliminary search of the prior art revealed the following prior U.S. patents of interest in connection with the present invention: Marcovecchio et al, U.S. Pat. No. 3,442,593; Decker, U.S. Pat. No. 3,578,980; Itoi et al, U.S. Pat. No. 4,063,817; Riccardi et al, U.S. Pat. No. 4,178,102; Maeda et al, U.S. Pat. No. 4,180,327; Lubbers et al, U.S. Pat. No. 4,255,053; Sengoku et al, U.S. Pat. No. 4,299,487; Sagusa et al, U.S. Pat. No. 4,318,615.